1. Field of the Invention:
This invention relates to a control unit for a welding apparatus, and particularly one that controls the heat input at the welding site in accordance with the wire feed rate and is adapted to be manually adjusted to accommodate different types of wires and welding processes.
2. Background Discussion:
In the art of welding it is conventional to use electric power as a source of heat. A specific type of wire as determined by the particular welding process is fed to the workpiece and an electric current is applied to the wire to create an arc between the wire and the workpiece when the wire is brought into proximity to the workpiece. The level of electric power, that is, the current flowing between the wire and workpiece, is determined by number of parameters, including the type and thickness of workpiece, the shielding gas which is employed to shield the wire and workpiece from the surrounding atmosphere, and the wire feed rate. If these parameters are not properly balanced, the weld will be defective.
One welding technique employs pulses of electric current to heat the wire. In pulse welding, metal transfer only occurs during current pulses. The duration of the pulses is determined by the wire diameter and type of welding process, for example, welding different workpieces such as steel or aluminum. The heat into the weld is controlled by the rate at which the pulses occur. The pulse repition rate determines the pulse period, the interval between respective points on the pulse. The heat input may then be defined as the percent of the pulse period during which the pulse is "on." For example, if the pulse is on 2 milliseconds out of an 8 millisecond period, the pulse is on 25 percent of the time. When the pulse is not "on," a background current is usually applied to maintain the arc and to keep the workpiece hot, without causing metal transfer from the wire.
For a given welding process, the rate at which the wire is fed to the workpiece will vary depending upon the objectives and skill of the workman doing the welding. If a thick work piece is being welded, for example, the rate at which the wire is fed to the workpiece will be increased, requiring more current or heat input. Alternatively, a larger diameter wire could be employed. In some instances, welding is conducted by robots which move the welding torch at a specified rate relative to the workpiece and the wire feed rate. The type of wire and shielding gas are selected for the given workpiece and the desired weld. If the workman decides to move the welding torch at a greater rate of speed or, if the robot is reset to move the torch at a greater rate of speed than initially established, the wire feed rate must be increased. An adjustment in the power supply to increase the current being applied to the wire must be made, requiring both skill and experience. Two adjustments must be made. One for the wire feed rate, and the other for the heat input or current. Sometimes two workman must be employed to achieve these two adjustments.